The use of dipyridoxyl based chelating agents and their metal chelates and certain manganese containing compounds, in particular manganese chelates, in medicine is known. See EP 0910360, U.S. Pat. No. 6,147,094, EP 0936915, U.S. Pat. No. 6,258,828, EP 1054670, U.S. Pat. No. 6,310,051, EP 1060174, and U.S. Pat. No. 6,391,895, for example, which disclose that certain chelating agents, in particular dipyridoxyl and aminopolycarboxylic acid-based chelating agents, and their metal chelates, are effective in treating or preventing anthracycline-induced cardiotoxicity, radiation-induced toxicity, ischemia-reperfusion-induced injuries and atherosclerosis, or from a more general point of view, every pathological condition caused by the presence of oxygen-derived free radicals, i.e., oxidative stress, in humans and animals.
Short-lived but highly reactive oxygen-derived free radicals have long been known to participate in pathological tissue damage, especially during treatment with cytotoxics/cytostatics and radiotherapy in cancer patients (Towart et al., Arch Pharmacol 1998; 358 (Suppl 2):R626, Laurent et al., Cancer Res 2005; 65:948-956, Karlsson et al., Cancer Res 2006; 66:598, Alexandre et al., J Natl Cancer Inst 2006; 98:236-244, Doroshow, J Natl Cancer Inst 2006; 98:223-225), acetaminophen-induced liver failure (Bedda et al., J Hepatol 2003; 39:765-772; Karlsson, J Hepatol 2004; 40:872-873), in ischemic heart disease (Cuzzocrea et al., Pharmacol Rev 2001; 53:135-159) and in various neurodegenerative diseases, including Alzheimer's disease, amyotrophic lateral sclerosis (ALS), Parkinson's disease, and multiple sclerosis (Knight, Ann Clin Lab Sci. 1997; 27:11-25). Overproduction of oxygen-derived free radicals is also implicated in pathological conditions of iron overload (Rachmilewitz et al., Ann N Y Acad Sci. 2005; 1054:118-23), for example, in thalassemia, sickle cell anemia and transfusional hemosiderosis. Oxygen-derived free radicals are also implicated in hepatitis-induced liver cirrhosis (Farrell et al., Anat Rec 2008; 291:684-692) and in noise-induced hearing loss (Wong et al., Hear Res 2010; 260:81-88).
One of the MnPLED-derivatives, namely manganese N,N′-bis-(pyridoxal-5-phosphate)-ethylenediamine-N,N′-diacetic acid (Manganese DiPyridoxyl DiPhosphate; MnDPDP), is approved for use as a diagnostic MRI contrast agent in humans. Interestingly, MnDPDP has also been shown to protect mice against serious side effects of several cytotoxic/cytostatic drugs (doxorubicin, oxaliplatin, 5-fluorouracil and paclitaxel), without interfering negatively with the anticancer effects of these drugs (Towart et al., 1998, Laurent et al., 2005, Karlsson et al., 2006, Alexandre et al., 2006, Doroshow, 2006). MnDPDP has been tested in one colon cancer patient going through palliative treatment with a combination of folinate, 5-fluorouracil and oxaliplatin (FOLFOX) (Yri et al., Acta Oncol. 2009; 48:633-635). The preclinical data and the results from this single patient were so promising that clinical testing in cancer patients has started in Sweden. A first feasibility study has been completed and positive results have been reported to Swedish Medical Agency.
MnDPDP has also been described to protect mice against acetaminophen-induced acute liver failure in mice (ALF) (Bedda et al., 2003; Karlsson, 2004). ALF is characterized by massive hepatocyte cell death, a condition caused by glutathione depletion, oxygen-derived free radicals and mitochondrial damage.
MnDPDP is a pro-drug in the sense that it probably has to be metabolized into N,N′-dipyridoxyl ethylenediamine-N,N′-diacetic acid (MnPLED) before it can exert cytoprotective effects during in vivo conditions (e.g., see Karlsson et al., Acta Radiol 2001; 42:540-547).
Manganese is an essential as well as potentially neurotoxic metal. It has been know for many years that under conditions of chronic exposure to high levels of manganese, a syndrome of extrapyramidal dysfunction similar to Parkinson's syndrome, although clinically a different disease entity, frequently occurs (see Scheuhammer & Cherian, Arch Environm Contam Toxicol 1982; 11:515-520). When a diagnostic MR imaging dose of MnDPDP is intravenously injected into humans, about 80% of the administered manganese is released (Toft et al., Acta Radiol 1997; 38:677-689). Release of paramagnetic manganese is in fact a prerequisite for the diagnostic MR imaging properties of MnDPDP (Wendland, NMR Biomed 2004; 17:581-594). On the other hand, the therapeutic effects of MnDPDP and its dephosphorylated counterparts MnDPMP (N,N′-dipyridoxylethylenediamine-N,N′-diacetate-5-phosphate) and MnPLED depend on the intact metal complex (Brurok et al., Biochem Biophys Res Commun. 1999; 254:768-721, Karlsson et al 2001; 42:540-547).
PLED-derivatives mimic the mitochondrial enzyme manganese superoxide dismutase (MnSOD) (Brurok et al., 1999). MnSOD protects the mammalian cell from the superoxide radical, a byproduct from oxygen metabolism, which is produced in fairly high amounts during normal aerobic conditions; no mammalians survive without a functional MnSOD. MnSOD has the fastest turnover number (reaction rate with its substrate) of any known enzyme (>109M−1 s−1) (Fridovich, J Exp Biol. 1998; 201:1203-1209). Low molecular weight MnSOD mimetics may have turnover rates close to that of native MnSOD (Cuzzorea et al., 2001). Interestingly, physiological buffers containing transition metals like manganese may have similar high turnover numbers (Culotta et al., Biochim Biophys Acta. 2006; 1763:747-758). However, the importance of native SOD enzymes is consistent with a selection process favoring organisms that elaborate a means of localizing transition metal catalyst for superoxide dismutation to parts of the cell where there is a high need for such dismutation, e.g., mitochondria. Furthermore, results from myocardial ischemia-reperfusion in anaesthetized pigs inevitably show that the intact MnPLED, but not manganese per se, protects against oxidative stress, seen as reduction in infarct size (Karlsson et al., 2001). Effective inactivation of superoxide is essential in preventing generation of very devastating hydroxyl radicals and peroxynitrite (Cuzzocrea et al., 2001). During pathological oxidative stress, the formation of superoxide radicals often exceeds the endogenous capacity for inactivation. Furthermore, superoxide stimulates production of peroxynitrite which nitrates endogenous SOD. Once nitrated, MnSOD and/or CuZn SOD lose their enzymatic activity, an event favoring the accumulation of superoxide and superoxide-driven damage (Muscoli et al., Br J Pharmacol 2003; 140:445-460). Exogenous addition of MnPLED-derivatives may in such situations re-establish the protective potential. PLED-derivatives are in addition strong iron binders, as described in EP 1054670, U.S. Pat. No. 6,310,051 and by Rocklage et al., (Inorg Chem 1989; 28:477-485), and some MnPLED-derivatives may have catalase and glutathione reductase activities (Laurent et al., 2005), which may further increase their antioxidant capacity.
For diagnostic imaging use and other sporadic use, dissociation of manganese from MnDPDP represents no major toxicological problem. Due to uptake into CNS, however, for more frequent use, for example in therapeutic methods, accumulated manganese toxicity may represent a serious neurotoxicological problem (Crossgrove & Zheng, NMR Biomed. 2004; 17:544-53). Thus, for more frequent therapeutic use, compounds that readily dissociate manganese should be avoided.
In order for manganese to distribute from blood into brain tissue, it must cross either the blood-brain barrier or the blood-cerebrospinal fluid barrier. The mechanism by which manganese is taken up by the brain is poorly understood. However, some references suggest that manganese is taken up as a free ion (Mn2+/Mn3+) or as manganese citrate and support the hypothesis that manganese transport is facilitated by either an active or a passive mechanism (Rabin et al., J Neurochem. 1993; 61:509-517; Yokel, Environ Health Perspect 2002; 110 suppl 5:699-704). Manganese may also be transported into CNS bound to transferrin. Nevertheless, in the case of MnDPDP and its dephosphorylated counterparts (in addition to other MnPLED-derivatives), manganese must probably dissociate from its corresponding chelator DPDP, DPMP or PLED (or other PLED-derivatives) to gain access into the brain.
Treatment with the metal chelator EDTA in rats who have previously been systemically exposed to manganese for many days considerably increased urinary excretion of manganese (Scheuhammer & Cherian, 1982) Similar effects of EDTA have also been seen on urine concentration of manganese in chronically poisoned welders (see Crossgrove & Zheng, 2004). Treatment of rats with manganese (II) chloride (50 mg/kg body weight, i.p.) once daily for 1 or 4 days led to increases in manganese levels of up to 232, 523, and 427% in the cerebral cortex, globus pallidus, and cerebellum, respectively. These changes were accompanied by development of pathological changes in glial morphology. Co-treatment with the manganese chelator 1,2-cyclohexylenedinitrilotetraacetic (CDTA) completely blocked this pathology (see Hazell et al., Neurosci Lett. 2006; 396:167-71), although these authors did not report if this effect of CDTA was due to direct inhibition of manganese uptake into the brain.
Thus, while manganese complex compounds are known for providing therapeutic effects in various treatments, there is a need to develop means for obtaining such therapeutic effects while reducing the undesirable side effects associated with such treatments.